key-spacing fixable documentation note

[RRMoelker]

The key-spacing rule is fixable but is not noted as such in the documentation. The fixer was introduced in #6169 .

I suggest changing the following two Markdown files:

• https://github.com/eslint/eslint/blob/master/docs/rules/key-spacing.md , insert a notice about it being fixable at the top (like https://github.com/eslint/eslint/blob/master/docs/rules/space-in-parens.md)
• https://github.com/eslint/eslint/blob/master/docs/rules/README.md , add (fixable)

I'll link PR asap.